1. Field of the Invention
This invention relates to an apparatus and method of at least partially demetallizing a metallized film. More particularly, this invention relates to an apparatus and method of selectively demetallizing a metallized film that utilizes a heat transfer roller to heat a metallized film prior to, during and after the application of a caustic solution.
2. The Prior Art
Thin metallic films are commonly used in the conversion of microwave energy, incident upon such a film, to thermal energy useful, for example, in the heating of food. It has been found that microwave energy is more efficiently converted to useful thermal energy within metallic films, such as aluminum, than within common food materials. The material to be heated is placed in close proximity, or in near contact, to such a film, so that the thermal energy produced within the film is efficiently conducted to effectively heat the material. The metallic films are typically prepared as a uniform and thin distribution of a metal on a plastic support film, such as polyester or mylar.
In some food applications, such as the microwave heating of popcorn kernels, the spatial heating pattern needs to be controlled for optimal cooking time convenience and food quality. The spatial heating pattern may be controlled by the use of a non-uniform metal distribution on a plastic support film in near contact with the food. A non-uniform metal distribution might have metallized and non-metallized areas in a pattern experimentally determined optimal. Heating is then selectively affected near the metallized areas. A non-uniform metal distribution upon a plastic support film may be produced through the selective demetallization of a uniform distribution. Thus, the convenience and quality of microwavable food, such as popcorn, may rely on the technology of the patterned demetallizing of otherwise uniformly metallized plastic films.
As known to people skilled in the art, currently there are several methods of demetallizing a metallized surface in a desired pattern using etchant solutions. One method is to print a patterned barrier coating, like a mask, on the surface areas where metal is to remain. The masked surface is then exposed to a highly caustic solution that removes the metal wherever the barrier coating is not present. The caustic solution is then typically rinsed from the surface and, after rinsing, the surface is dried prior to further processing. Elevated temperature and elevated etchant solution concentration each serves to speed the etching process. However, the latter involves the problematic handling of highly corrosive materials. Thus, it is preferred that the etching process occur at an elevated temperature.
U.S. Pat. No. 4,517,045 to Beckett discloses an apparatus and method for printing a barrier coating on the areas of a metallized film where metal is to remain throughout an etching process. An etchant-resistant barrier material is applied to a patterned roller which transfers then a patterned barrier material to a metallized film. A sodium hydroxide etchant solution is then applied to the film from a wet roller. Etching occurs at the areas unprotected by the patterned barrier material. It is disclosed that etching is preferably affected by the application of heated sodium hydroxide etchant. This is accomplished according to the teaching of Beckett by heating the wet roller, thus the etchant is hot when applied to the metallized surface. This type of selective etching method may suffer several problems. The application of a patterned barrier material to the metallized film is a step that may be avoided to simplify the process. The direct heating of the etchant solution, on the wet roller, may cause undesirable drying which does not efficiently serve the needs to apply the solution to the metallized film. Furthermore, the etched film must then be rinsed and dried prior to further processing.
Another method of selectively demetallizing a uniform metal distribution on a plastic backing film does not include the ultimate removal of the selected metal from the film. A selected break-up of the uniform distribution has been found to be sufficient for the control of spatial heating under microwave radiation. A metal may become distributed non-uniformly into tiny islands upon the backing film, and such an arrangement does not provide for the efficient conversion of radiant microwave energy to useful thermal energy.
As disclosed in U.S. Pat. No. 4,685,997 to Beckett, a chemical etchant pattern is printed on a first polymer film which is then laminated onto a metallized second polymer film. The etching occurs after lamination without the need for elevated temperatures. However, this method may have a number of disadvantages. First, in the preferred embodiment the etchant is printed on a web that is subsequently laminated to the metal-coated surface of an adjacent web. While this is possible, it means that the etchant becomes the “adhesive” to mechanically bond the sheets together with strength adequate to sustain the secondary uses of the laminate such as forming and filling a finished package. Furthermore, a method of bonding the remaining metallized surface must be provided. Bonding the untouched metal surface must be accomplished without adding a barrier between the caustic and the metal. Still further the invention claims to be useful in laminating two non porous sheets with aqueous “solutions” while still claiming that the etchant need not be dry before the webs are brought together. This process is known as wet bonding and is used only when one or both sheets are porous enough to allow the laminating medium to dry. Still another disadvantage is that when the (wet) demetallizing solution is pressed onto the adjacent sheet, the previously undisturbed pattern may squeeze out resulting in loss of the intended pattern.
In another example, Beckett describes printing the etchant solution directly on the metal. This method may have the same drawbacks. If one attempts to overcome these drawbacks by providing at least one web of a porous material two results are likely. First, if the etchant is printed on the porous web it will be absorbed into the sheet allowing little or no etchant on the surface to demetallize its adjacent sheet. Second, if the etchant is printed on the non-porous metallized sheet the etchant will be neutralized by the overall lamination adhesive as the sheets are brought together.
Another demetallizing method that is taught in U.S. Pat. No. 4,959,120 to Wilson. A major differentiation is that Wilson does not use the barrier coating. The etchant is printed on the areas where metal is to be removed. The etchant is then allowed enough time to remove the metal. The next step is to rinse the etchant from the film to provide a clean surface that can later be laminated or printed. Again, one of the last steps involves drying the film after rinsing. In addition to the drawbacks in drying the film this process has an additional flaw. The highly caustic etchant actually washes over non-barrier coated metal areas that should not be disrupted. This process typically uses a high pH solution that is printed with conventional printing methods and such hot “inks” do not print well because the “caustic ink” tends to dry on plates, rollers, etc. before being transferred to the substrate. Furthermore, the rinsing operation becomes very critical because the caustic typically has come in contact with the areas that are not to be demetallized. Any etchant left on the metal areas because of rinsing can continue to eat the metal away for days or weeks after the product has been produced.
What is needed then, is a method of manufacturing patterned metallized films in a rapid and cost efficient manner. A method utilizing heated demetallizing with an etchant should provide rapid manufacturing and avoid the problematic handling of high-concentration etchants. Further, as in all manufacturing, simple methods using only a minimal number of steps should provide cost efficiency by lowering equipment costs. Specifically the rinsing of an etchant from a patterned demetallized film is preferably avoided.